[NFC][CLANG] Fix nullptr dereference issue in DeduceTemplateArgumentsByTypeMatch()

[Manna]

Summary

DeduceTemplateArgumentsByTypeMatch() returns null value which is dereferenced without checking since
getAsIncompleteArrayType() returns nullptr and we are dereferencing null pointer value for S.Context->getAsIncompleteArrayType(P) when calling getElementType().

This patch adds an assert.

[erichkeane]

1 nit to do on commit.

clang/lib/Sema/SemaTemplateDeduction.cpp

1707

[Manna]

Thank you @erichkeane for reviews!

I have updated assert message.

clang/lib/Sema/SemaTemplateDeduction.cpp

#include "clang/AST/TypeNodes.inc"
     }
 
     //     T [] (implied, but not stated explicitly)
     case Type::IncompleteArray: {
       const auto *IAA = S.Context.getAsIncompleteArrayType(A);
       if (!IAA)
         return Sema::TDK_NonDeducedMismatch;
 
+      const auto *IAP = S.Context.getAsIncompleteArrayType(P);
+      assert(IAP && "Template parameter not of incomplete array type");
+
       return DeduceTemplateArgumentsByTypeMatch(
-          S, TemplateParams,
-          S.Context.getAsIncompleteArrayType(P)->getElementType(),
-          IAA->getElementType(), Info, Deduced, TDF & TDF_IgnoreQualifiers);
+          S, TemplateParams, IAP->getElementType(), IAA->getElementType(), Info,
+          Deduced, TDF & TDF_IgnoreQualifiers);
     }
 
     //     T [integer-constant]
     case Type::ConstantArray: {
       const auto *CAA = S.Context.getAsConstantArrayType(A),
                  *CAP = S.Context.getAsConstantArrayType(P);
       assert(CAP);
       if (!CAA || CAA->getSize() != CAP->getSize())